The present invention relates to a tire that is excellent especially in performance on snow or ice road.
Conventionally, when driving an automobile on a snow road in winter, safety on snow or ice road was secured by using studded tires having spikes thereon or tire chains mounted around outer periphery of tire. However, road is apt to be worn and damaged by using the studded tires or the tires having tire chains, and they forms dust particles to bring about pollution and cause a great environmental problem. Therefore, to solve the safety problem and the environmental problem, studless tires have been widely used.
As a technique for improving the performance of the studless tire on ice or snow road, Japanese Unexamined Patent Publication No. 4-110211/1992, for example, discloses a technique in order to improve the frictional force on ice road, i.e., to improve adhesive force and frictional force at a low temperature. The method is a method that non-metal staple fibers such as cellulose staple fibers are mixed with a rubber compound which is excellent in low temperature characteristics, i.e., flexible even at a low temperature, and rigidity at a tire block is controlled by an elasticity difference of the rubber at a block surface, a wall surface portion and an inside portion of the tire generated by orientation of the staple fiber along a block portion surface of a tread portion, and thereby attempting to improve the adhesion force and the friction force to the road surface. However, if the staple fibers are mixed with the tread rubber and a tire is formed by extrusion, the mixed staple fibers are oriented along the machine direction, i.e., along a circumferential direction of the tire. And if the composition is molded and vulcanized into a tire, the most staple fibers of a tread rubber contacted to road surface are oriented in the circumferential direction of the tire, which reinforces and hardens the rubber surface to lower the adhesion of the tire on the contrary when the tire is contacted with an uneven spot on the road.
Since most of the staple fibers are oriented in the circumferential direction of the tire, there are problems that effect for digging and scratching friction is not sufficient. And abrasion resistance becomes low, if further staple fibers are mixed.
Japanese Unexamined Patent Publication No. 2-274602/1990 discloses a studless tire and a method for preparing the same, in which powders having relatively large particles or staple fibers are oriented not in the circumferential direction of the tread but in the thickness direction. This is a technique for improving digging friction by scratching the ice and snow road surface with the particles and the staple fibers, but it is necessary to protrude the staple fibers and particles from the tread rubber surface for the digging effect. Steel staple fibers are used in a disclosed example. But if hard staple fibers such as metal are mixed, the rubber is hardened, abrasion resistance between the rubber and the metal is largely different, and contact area is lowered by disturbing contact of the rubber with road surface according to the protruded metal staple fibers to reduce adhesion force and friction force, and thereby the performance on ice and snow road is deteriorated.
As described above, there exists no tire, which improves or balances adhesion, adhesion friction, digging friction, scratching friction and abrasion resistance, and has excellent performance on ice and snow road.
The present invention has been carried out for solving these problems.